Mechanisms of the above-defined type are already known. For example, there is disclosed in French Pat. No. 2,496,193 a mechanism whose decompression chamber is connected to a low pressure discharge reservoir by pipe means having a solenoid or electromagnetic valve. The control bar is lifted by opening the electromagnetic valve so as to create a pressure differential which applies an upwardly directed force to the rod. The means for mechanically securing the rod in the high position are formed by a bi-stable toggle device whose condition changes position upon each opening and closure cycle of the electromagnetic valve.
An advantage of such a mechanism is simplicity in design; on the other hand, it has drawbacks. A first drawback is that it is not possible to control, at will and independently, the force tending to lift the bar and the rising speed of the bar. In fact, both values are increasing functions of the degree of opening of the electromagnetic valve and a decreasing function of the leaks between the rod and the cylinder. It is, for example, not possible to increase the lifting force while reducing the speed. In addition, the relationship which exists between the degree of opening of the electromagnetic valve and the force (or the speed) varies over time, especially due to wear or clogging.
Another drawback is related to the fact that reactors typically include several tens of control bars. If each drive mechanism comprises pipe means and an electromagnetic valve connected to a low pressure dump reservoir, a number of pipe lengths must be provided which, since they convey a flow of primary coolant, must comply with very strict safety rules, and must be protected against earthquakes and possible missiles. The pipe lengths and their electromagnetic valves are potential sources of leaks likely to cause uncontrolled lift of control bars.